An electrostatic focused electron image device

ABSTRACT

An electron image device having an electron optical system including a photocathode element capable of converting a radiation image into an electron image. The electron image generated by said photocathode is focused by a cylindrical focusing system onto a substantially flat target surface. The cylindrical focusing system includes an anode electrode positioned intermediate said photocathode and said target and is the primary electrode for a main lens for focusing the electron image onto the target. A distortion correction electrode is positioned intermediate the anode and the target for correcting distortion within the image. The distortion correction electrode is electrically isolated from both the anode and the target and is provided with an adjustable voltage source for correcting the distortion within an image under different operating conditions.

United States Patent Santilli et al.

1 51 Aug. 29, 1972 [54] AN ELECTROSTATIC FOCUSED ELECTRON IMAGE DEVICE FOREIGN PATENTS OR APPLICATIONS 971,495 9/1964 Great Britain ..250/213 [72] Inventors: Vincent J. Santilli, 159 E. 2nd St.,

Corning, NY. 14830; James Vine, Primary Examiner-Walter Stolwein 100 Biddle Ave., Pittsburgh, Pa. Attorney-F. H. Henson and C. F. Renz 15221; James W. Cottennan, 100 Bowman Hill Terrace, Horseheads, T T 14845 An electron image device having an electron optical [22] Filed; April 16, 1968 system including a photocathode element capable of converting a radiation image into an electron image. [2]] Appl' The electron image generated by said photocathode is focused by a cylindrical focusing system onto a sub- 52 us. (:1. ..2s0/213 VT, 313/65 R Stamiallly flat target surface The cylindrical focusing 51 Int. Cl ..H01j 31/50 system includes an anode electrode Positioned i 58 Field of Search ..250/213 VT, 227; 313/65 R, Smd Photocathode e sald target 18 the 313/65 A, 65 3 primary electrode for a main lens for focusmg the electron image onto the target. A distortion correction 56] Referem Cited electrode is positioned intermediate the anode and the target for correcting distortion within the image. The UNITED STATES PATENTS distortion correction electrode is electrically isolated from both the anode and the target and is provided 3122 322 211222 "3224212; m w 3267283 8/1966 p y X distortion witlun an image under dlfierent operatmg d't' 3,274,416 8/1966 Rotow ..250/213 x 3,480,782 11/1969 Burns ..250/213 VT 4 Claims, 2 Drawing Figures +2ov 82 32 so I I l 104: 72 5O 52 48 e1 10 l I l l I 1 r4 I 1 1 s 42 0 404 36 38 76 l 34 \I 58) i i 44/ PATENTED M1829 m2 INVENTORS Vincent J. Sontilli, James Vine rQv TORNEY 8 James W. Cottermon III/III!) III/II v mm AN ELECTROSTATIC FOCUSED ELECTRON IMAGE DEVICE The invention herein described was made in course of or under a contract or subcontract thereunder with the Department of the Army.

"BACKGROUND OF THE INVENTION There are several image devices known in the art in which an electrostatic focusing system is utilized for transforming an optical image into an electrical signal, or an amplified or converted optical output image. In these devices, an electrostatic image section is provided wherein the input optical image is focused onto an input faceplate. This faceplate may be of glass or fiber optics. A photocathode capable of emission of photoelectrons in response to the input radiations is provided on the inner surface of this input faceplate. The photoelectrons emitted from the photocathode are electrostatically focused onto a target. The target may take the form of a fluorescent output screen or may be of some type of storage material capable of storing the electron image from the photocathode and converting this into an electrical signal by suitable means. An electrostatic imaging system is described in a copending application Ser. No. 560,385,filed June 23, 1966 entitled Electron Image Device by James Vine. It is found that the system described therein does provide high resolution but does suffer image distortion. It is found that the surface of unity magnification is a concave surface toward the photocathode. In most applications, the target is in the form of a flat surface and the resulting image suffers from distortion in that there is a nonuniformity of magnification radially out from the center of the image. It is found that the magnification difference from the center to the edge may be as great as 20 percent. The magnification increases from the center to the edge and therefore the result is a pin cushion type of distortion.

It is accordingly a general object of this invention to provide a distortion correction electrode within the electrostatic focusing system to remove the distortion and provide linear magnification over the output target surface. It is also a general object to provide correction in a small tube volume.

SUMMARY OF THE INVENTION This invention is directed to the removal of distortion for an electrostatic imaging device. This is accomplished by providing a distortion correction electrode within the region of the electrostatic lens system between the anode and the target surface. By providing an electrode which is electrically isolated from the anode and the target electrode, a control voltage may be applied to the distortion correction electrode so as to provide a substantially distortion-free image.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a sectional view of a radiation sensitive pickup tube embodying the present invention; and

FIG. 2 is a schematic representation illustrating the electron paths within the imaging section of the device illustrated in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a pickup tube 10 is illustrated. The tube 10 comprises an evacuated envelope l2 having an image section 14 and a readout section 16. A target structure 20 separates the image section 14 and the readout section 16. The end of the image section 1 1 remote from the target 22 is closed off by an input window 22. The end of the readout section 16 remote from the target 22 is closed off by a base portion 24.

The image section 14 consists of the input window 22 which may be of a suitable material transmissive to the input radiations. In the specific embodiment shown, the window 22 is of a fiber optic glass structure in which the light pipes are perpendicular to the exterior plane of the input window 22. A photoemissive coating 26 is provided on the inner curved surface of the input window 22. A suitable material for the photoemissive coating 26 is a cesium antimony. A flange member 28 is secured to the input window 22 and is also utilized as an electrical conductor for applying a voltage to the photoemissive coating 26. The flange member 28 is in turn secured to a tubular metallic portion 30 forming a part of the envelope 12. A plurality of tubulations 32 are disposed in the tubular member 30 for evacuating the envelope 12 and evaporating the photoemissive coating 26. The tubular member 30 is provided with an inturned flange 34 to which a cathode shield or focusing electrode 36 is secured. The cathode shield 36 is of a smaller diameter than the tubular member 30 and is electrically connected to the photocathode 26 by means of the tubular member 30 and the flange 28. The cathode shield electrode 36 consists of a cylindrical portion 38, an inturned flange portion 40 is provided on the cathode shield electrode 36 on the remote side of the cylindrical portion 38 with respect to the photocathode 26. The flange portion 40 is provided with a lip portion 42.

A tubular insulating member 44 is secured to the flange portion 34 of the tubular member 30 with the other end of the member 44 secured to an anode flange member 46. The anode flange member 46 is of electrically conductive material and provides means of applying potential from the exterior of the envelope to an anode electrode 48 positioned within the envelope 12. The anode flange member 46 also supports the anode electrode 48. The anode 48 consists of cylindrical portion 50 with a conical portion 52 provided thereon with the smaller portion of the conical portion 52 facing the photocathode 26. The anode 48 is coaxial with the cathode shield electrode 36 and in the specific embodiment shown the conical portion 48 projects into the region defined by the focusing electrode 36. The photocathode 26, cathode shield electrode 36 and the anode 48 form the main focusing lens.

The next electrode positioned within the image section is a corrector electrode which is also cylindrical shape and is positioned on the remote side of the anode 48 adjacent to the tubular section 50. The corrector electrode 60 is of a larger diameter than the cylindrical portion 50 of the anode 48. An annular flange member 62 extends outwardly from the outer surface of the corrector electrode 60. An annular insulating member 64 forming a part of the envelope 12 is sealed to the corrector flange 62 and the anode flange 46. A cylindrical insulating member spacer 66 is secured to the opposite side of the corrector flange 62 and is in turn sealed to metallic flange member 68 which supports the target structure 20. The flange 62 is of electrically conductive material and provides means of supplying potential to the corrector electrode 60. A suitable variable potential source 65 is connected to the flange 62. The target structure 20 is positioned on the opposite side of the corrector electrode 60 with respect to the focusing anode 48. The target structure 20 may be of any suitable type responsive to electron bombardment and capable of storing a charge image which may then be readout by an electron gun. The target may also be nonstorage phosphor screen. In the specific device shown herein, the target 20 consists of a support layer 70, an electrical conductive layer 72 and a porous storage layer 74. A more complete description of this target member 20 may be found in U.S. Pat. No. 3,213,316, entitled Tube with Highly Porous Target by G. W. Goetze et al, issued October 1965. The support member 70 may be of any suitable material such as aluminum oxide and the electrical conductive coating 72 may be of a suitable material such as aluminum. The porous layer 74 may be of a suitable material such as potassium chloride in which the density of the deposit is less than percent of the normal bulk density. The layer 74 is extremely porous so that secondary electron conduction within the porous material is within the voids of the layer 74.

The target may be supported by a flange member 76 secured to the tubular member 68. This support assembly including the flange 76 and the annular member 68 also provide means of supplying potential to the suppressor grid electrode 80 provided in front of the porous layer 74 and facing the read section 16. The grid 80 provides means of maintaining a positive potential below the first crossover or breakdown potential of this target so that the exit surface of the layer 74 is limited to prevent destruction of the target. A suitable lead-in member 82 is provided through the envelope 12 and insulated from the annular member 68 to provide means of deriving a signal from the conductive coating 72. The output signal is derived from the electron tube 10 from the lead 82.

A suitable electron gun 90 is provided in the read section 16 of the electron tube for generating a small pencil-like electron beam for reading out the charge image on the target 20. The electron gun 90 may be of any suitable type capable of generating a small electron beam and a deflection means illustrated as at 92 is provided about the envelope for deflecting the read electron beam to scan the raster over the target 20 in a conventional means. In the specific embodiment illustrated, a filament 94 is illustrated for heating a cathode 96. The cathode 96 is a tubular element with a diaphragm across one end on which an electron emissive coating is provided for generating electrons in a conventional manner. A control grid 98 is provided in front of the cathode 96 and includes a tubular member with a diaphragm having an aperture therein located in front of the cathode 96. A second grid 100 may be provided in front of the control grid 98 and may include two diaphragms with apertures therein. A final anode 102 comprises. a tubular member having a diaphragm adjacent the second grid 1100 with an aperture therein.

A collector mesh 104 may be provided at the opposite end of the anode 102. Suitable voltages are applied to the electron gun by means of the lead-in members 106 passing through the tube base 24.

In the general operation of the tube, a light image is directed onto the photocathode 26 which operates at a potential of about 8,000 volts negative with respect to ground and photoelectrons are generated. These photoelectrons are focused by means of the main lens including the cathode shield electrode 36 which operates at the same potential as the photocathode 26. The anode electrode 48 operates at a potential of about ground. These photoelectrons are directed onto the target 20 by the main lens. The conductive coating 72 of the target is at a potential of about 20 volts positive with respect to ground. The corrector electrode 60 is operated at a potential of about 4,000 volts negative with respect to ground. The voltage may range from a negative 8,000 to ground. Selection of the proper voltage corrects distortion within the image which will be described in more detail later. The photoelectrons directed onto the target 20 generate a charge image on the surface of the porous layer 74 and this image is read out by means of the electron gun 90 by scanning a suitable raster by means of the deflection coil 92. A video signal will be obtained from the lead-in 82 representative of the video information within the image directed onto the target 20 and this video signal may be reconstructed in well known manner.

In the specific embodiment described herein, an electron beam is utilized in conjunction with the output target 20 to derive an electrical output. It is of course obvious that any kind of a flat output target could be utilized with the image section 12. For example, a phosphor screen might be utilized in place of the target 20 to give a visual output. The structure of the image section in the specific device illustrated consists of the following dimensions Photocathode to target 2.55 inch Radius of curvature of photocathode .70 inch Photocathode to anode (48) tip .785 inch Anode aperture .l20 inch Corrector electrode diameter 1.47 inch Target diameter 1.00 inch Length of cathode shield electrode .75 inch Length of anode 1.30 inch Length of corrector electrode .385 inch Diameter of cathode shield electrode 1.45 inch It is found with the above dimensions and voltages and without the corrector electrode that a unity magnification surface illustrated as the dotted line 114 in Fig. 2 is formed. This surface 114 is a concave surface and the output surface of the target 20 is a plane. The result is that a pin cushion type of distortion image is obtained when this focusing structure is utilized with a planar output target. By incorporating the corrector electrode 60 and operating this electrode at a potential of about negative 4,000 volts with respect to ground an electric force is applied in such a way as to bend the photoelectron paths at their end points toward the center of the field of view or the axis of the image section. This is primarily a third-order focusing effect. This is illustrated in Fig. 2 by the electron paths designated as numeral 116. It is found in this manner by utilizing the corrector electrode 60 that a planar unity magnification surface may be attained upon the surface of the target 20 thus removing the distortion from the image.

The distortion is an image defect which is sometimes referred to as a third order effect based on theory developed by Ludwig von Seidel. This effect deals with the electron rays making angles with the axis that are oblique to distinguish from the substantially paraxial rays. The distortion is one of the image defects which is found in an electrostatic focusing system. Distortion is a non-uniformity in the magnification radially from the axis. This non-uniformity of magnification leads to the well known pin cushion distortion where increased magnification is found with radial image distance. While the converse variation of magnification that is decreasing leads to the barrel distortion the corrector electrode 60 is specifically designed and operated to correct for this third order effect and the first order effect (converging lens effect) is reduced to a low level.

While the preferred embodiment of the invention has been disclosed herein, it is obvious that other modifications are feasible within the spirit and scope of the invention.

We claim as our invention:

1. An electrostatic focused electron optical converter system comprising an envelope, said envelope comprising an input window transmissive to input radiations, a photoemissive cathode positioned on said input window to receive radiations transmitted through said window, a target member positioned within said envelope to receive the photoelectrons from said photocathode, a main electrostatic field shaping electrode system positioned intermediate said photocathode and said target for focusing said electron image produced by said photocathode onto said target,

and a corrector electrode having a length less than its diameter positioned between said field shaping electrode system and said target and electrically isolated therefrom to reduce distortion within the electron image directed onto target member.

2. The system set forth in claim I in which means is provided for maintaining said corrector electrode at a potential negative with respect to said target and the adjacent electrode of said field shaping electrode system.

3. The system set forth in claim 1 in which said corrector electrode is a tubular member of a diameter such that it does not intercept said electron image and means for applying a potential to said corrector electrode to modify the uniform magnification surface of the electron image from a concave surface facing said cathode to a planar surface on the surface of said target and with minimum converging effect of the said electron image.

4. The system set forth in claim 1 in which said main field shaping electrode system includes a curved photoemissive surface convex with respect to the input window, a cylindrical cathode shield adjacent said photocathode, an anode electrode positioned between said photocathode and said target, said anode comprising a conical portion with the smaller end facing said photocathode, said corrector electrode is a cylindrical member positioned between said anode and said target and means for applying a potential to said correction electrode different from the potential applied to said target and said anode. 

1. An electrostatic focused electron optical converter system comprising an envelope, said envelope comprising an input window transmissive to input radiations, a photoemissive cathode positioned on said input window to receive radiations transmitted through said window, a target member positioned within said envelope to receive the photoelectrons from said photocathode, a main electrostatic field shaping electrode system positioned intermediate said photocathode and said target for focusing said electron image produced by said photocathode onto said target, and a corrector electrode having a length less than its diameter positioned between said field shaping electrode system and said target and electrically isolated therefrom to reduce distortion within the electron image directed onto target member.
 2. The system set forth in claim 1 in which means is provided for maintaining said corrector electrode at a potential negative with respect to said target and the adjacent electrode of said field shaping electrode system.
 3. The system set forth in claim 1 in which said corrector electrode is a tubular member of a diameter such that it does not intercept said electron image and means for applying a potential to said corrector electrode to modify the uniform magnification surface of the electron image from a concave surface facing said cathode to a planar surface on the surface of said target and with minimum converging effect of the said electron image.
 4. The system set forth in claim 1 in which said main field shaping electrode system includes a curved photoemissive surface convex with respect to the input window, a cylindrical cathode shield adjacent said photocathode, an anode electrode positioned between said photocathode and said target, said anode comprising a conical portion with the smaller end facing said photocathode, said corrector electrode is a cylindrical member positioned between said anode and said target and means for applying a potential to said correction electrode different from the potential applied to said target and said anode. 